How long to keep old domain for 301 redirects after moving WordPress?

Question

I'm looking into moving my WordPress blog from an old domain to another domain that I own. The old domain is getting expensive at $25/year and I have to renew in a month. I had the old domain for 15 years but it no longer serves it's purpose as a standalone blog.

I have several options for handling this:

1. Leave it as is and keep paying the renewal registration.

2. I can do the move right now and let the registration expire next month.

3. Make the move and renew for another year before letting the registration expire.

How long do I need to keep the old domain up for the 301 redirects to be effective before I let the registration expire?